DNA repair enzymes play a key role in maintaining the genomic integrity of all living cells. They are responsible for detecting and excising the potentially mutagenic lesions that arise in DNA either spontaneously or via oxidative damage. Detailed x-ray crystal structures of these enzymes and their DNA complexes, will provide a solid basis toward understanding the mechanisms and biologic ramifications of essential repair processes and will assist in the design of pharmacologically significant compounds with potentially vast chemo- and radiotherapeutic applications. Crystallization trials with endonuclease IV and with different oligonucleotide complexes of all four enzymes are underway.